A physical quantity sensor has a sensing portion formed with a substrate such as a semiconductor substrate or a ceramic substrate. The sensing portion is held in a casing made of a resin, for example. The sensing portion detects a physical quantity, such as angular velocity, acceleration, and turning power, based on a change in an electrical capacitance, a voltage, or the like.
In such a physical quantity sensor, if a vibration of the casing is transmitted to the sensing portion, a noise contained in an output of the sensing portion increases. Therefore, a vibration isolation structure is needed between the sensing portion and the casing to reduce a relative vibration between the sensing portion and the casing.
For example, Japanese Patent Application Publication No. 2007-212174 describes an angular velocity sensor having a resin spring as a vibration isolation structure. The resin spring is located between a casing and a sensing portion. The resin spring supports the sensing portion to the casing to reduce a relative vibration between the sensing portion and the casing.
In fact, the sensing portion is small. In order to ensure sufficient vibration isolation performance for such a small sensing portion, the resin spring needs to have a small spring constant. However, high accuracy is required to form a resin spring with a small spring constant. Further, it is difficult to ensure sufficient structural strength of the resin spring.
Further, in the sensor of the publication, flexible bonding wires are employed for electrically connecting the casing and the sensing portion. If the casing and the sensing portion are displaced relative to each other, the bonding wires are deformed or strained, and hence structural strength and durability of the bonding wires are degraded. To ensure the structural strength and the durability of the bonding wires, it is considered to increase the stiffness of the bonding wires such as by increasing the outer diameter of the bonding wires. Such a measurement, however, will affect the vibration isolation performance, and thus it may be difficult to achieve desirable vibration isolation performance.